Trawling

Gear requirements

Pelagic trawls are the primary tools for:

validating species composition of acoustic backscatter

obtaining length and weight frequencies

age structure

reproductive status

Trawls are the best available method of obtaining relatively unbiased estimates of species and size composition (Simmonds et al. 1992).  The goal of trawling is to obtain catches that are representative of the species composition and the length-frequency distribution of organisms detected acoustically (McClatchie et al. 2000).  This is difficult to accomplish because all biological sampling methods are species- and size-selective.  There has been little work on trawl selectivity in the Great Lakes.

Trawling to identify specific aggregations or layers benefits from effective net mensuration instrumentation.  Net mensuration instrumentation includes:

door and wing sensors

third-wire sensors

depth sensors

temperature sensors

head- and footrope sensors 

It is important to maintain consistency in trawl procedures between and within surveys.  It is especially important to measure sampling depth.  Currently (2006), depth and temperature sensors are used during most Great Lakes surveys, and headrope, footrope, and wing sensors are used in Lakes Superior, Huron, and Michigan.

All Great Lakes surveys use conventional midwater trawls to obtain biological samples (Table 7).  Current trawls sample mid-water layers and schools of fish continuously throughout a deployment.  As a result, samples of deeper layers contain fish caught in shallow layers, because the net is open on descent and ascent.  Moving toward new opening-closing mid-water trawl gear will result in better characterization of targeted schools and layers, and will result in more accurate length-frequency distributions.  Trawls are selective and both small and large fish may be under-represented in the catch.  A cod-end mesh of 10-13 mm will likely select against fish smaller than 5 cm, but the selectivity is complicated because the mesh size is variable throughout the trawl.

The incorporation of a bottom trawl sample in addition to mid-water trawls is being considered by groups in the Great Lakes. 

Table 7.  Summary of Great Lakes trawls

Lake	Trawl size (m)	Cod end linear mesh (mm)
Lake Ontario	8 x 13	12.7 (stretch)
Lake Erie	6 x 6	10.0 (stretch)
Lake Michigan	6 x 9	12.7 (stretch)
Lake Huron	6 x 9	12.7 (stretch)
Lake Superior	Under consideration
Lake Champlain	Under consideration

Fish behavior in temperature gradients is predictable and will not change dramatically between years.  Therefore, the accumulated information from many years of depth- and temperature-stratified sampling can be used to help in target identification.  Relying on past experience is common practice, but would benefit from a formal analysis. 

Frequency, location, and timing

The number, locations, and timing of trawl sets are dependent on the objectives of the survey.  In the Great Lakes, target species are stratified by temperature.  Different temperature layers should therefore be sampled.  Most Great Lakes surveys are done at night when fish are dispersed. During the day, single-species or single-size schools may form and a single school may not be representative of the cluster of schools in an area.  For this reason, hauls should include more than one small school and more than one part of a large school.

Catch processing

In many cases, trawl catches are too large to sample in their entirety and must be sub-sampled.  Even when an entire haul is processed for species composition by weight and number, additional information such as age, fish length and sex cannot normally be taken from all captured specimens and must be determined using a random sub-sample.  Determination of the sub-sample size should be guided by statistical principles. 

 

Underwater video

Although not currently used in any Great Lakes acoustic surveys, underwater video and low ambient light level still-camera systems provide visual identification of species and have the potential to document behavior. 

Limitations of underwater video include small detection ranges and volumes (order of meters) and potential disruption of behavior.  Because of the limited light penetration in water, cameras must be positioned near the targets of interest and often, artificial lighting must be used.  These two factors complicate acquisition of visual data for species identification and potentially alter the behavior of the organisms.